The first lineage differentiation of cells during pre-implantation embryo development is critical for continued embryonic growth. Though several transcription factors (TFs) have been identified that are involved in this transition, a specific mechanism of regulation has yet to be determined. Previous studies in our laboratory have shown the TF Lef1 is involved in differentiation of mouse embryonic stem cells into trophoblastic stem cells through the Wnt signaling pathway. Lef1 is known to be involved in lineage determination of adult skin stem cells. As we observed that two isoforms of Lef1 were expressed at different stages of pre-implantation development, we hypothesized that Lef1 was interacting with the other well-established lineage differentiation TFs, Nanog, Oct4, and Cdx2, in these early embryos. At the blastocyst stage, no significant changes in mRNA expression were seen when siRNAs, specifically designed to knockdown Lef1 expression, were injected in early embryos; however, knockdown of Lef1 did not interfere with blastocyst formation.